Posts Tagged ‘tree line’

Since we are in the season of comparing charts, graphs and interpretations of the summer Arctic ice melt, it may be useful to pause and consider the history of Arctic temperatures in the Holocene. There is an abundance of data compiled by hardworking field researchers over the years. Before everybody got so excited about global warming, it was understood that the Arctic was considerably warmer in earlier parts of the Holocene than in the present. The evidence for these warmer periods seems to have been forgotten in an age when satellite data causes us to fixate on the last thirty years.

I have collected a short list of papers that indicate times during the mid-Holocene, and places in or near the Arctic, when it was warmer than the present. Some of these papers may also indicate warmer periods in the early or late Holocene, but I am concentrating primarily on the mid-Holocene in this post. Figure 1, below, shows the spatial distribution of areas covered by these papers. Click on the image to get a larger view. Figure 2 shows the times in the mid-Holocene that each paper says it was warmer than the present.

Figure 1. Numbers correspond to the journal articles that are listed below. They also correspond to the numbered lines in figure 2.

Figure 2. “Paper #” corresponds to the numbered journal articles listed below. The colored areas indicate the time periods in the mid-Holocene for which the papers indicate it was warmer than present.

The evidence that the Arctic was warmer in the mid-Holocene than it is now is compelling. At longitudes almost completely encircling the Arctic, palaeological proxies of all kinds speak from the past with the same message. Treelines moved in latitudes and elevations. Alkenone molecules produced from sun loving organisms in the top layer of ocean water recorded the temperature of the water and settled into the depths of the ocean, depositing their temperature record in the sediments. The pollens of various species of plants changed their ratios with changing temperatures and forest locations, drifted over lakes and settled to the bottom, leaving layer upon layer of temperature history. Choronomid midges, small insects that live out their short lives in just a few weeks, varied their physiology according to the temperature of their environment, and carried their temperature stories to lake sediments. Forest plant species came and went at temperatures rose and fell, leaving behind their seeds in successive layers of soil as positive reminders that they had been there.

These proxies, and others, strongly indicate that the arctic region was warmer around 5,000 years ago than it is today. Read the papers listed below to see the details.

Please feel free to criticize my interpretations of the papers, or to point out contradictory or complementary papers.

Kim, et. al., used alkenone-derived sea-surface temperature records from sediments from over 30 locations to derive temperature changes in the Pacific and the Atlantic Oceans during the Holocene. I have marked the locations of the five highest northern latitude cores, two above the arctic circle and three below it. Kim’s data for these cores covers only the last 7,000 years, rather than the entire Holocene. Nevertheless, the cores show temperatures clearly dropping to modern values over the last 7,000 years. The northern-most core (75N) shows a temperature drop of 4.4 degrees C since 7,000 years ago. Two other cores show temperature drops greater than 3 degrees C (3.3 and 3.8 degree drops at 57.8N, 8.7E and 57.7N, 7.1E respectively). The remaining two cores show temperature drops of 1.8 and 0.6 degrees C. Get copy here.

In this paper, Kultti, et. al., (2004b) looked at tree lines in Finnish Lapland and found “Results indicate that pine reached its maximum distribution between 8300 and 4000 cal. yr BP. The inferred minimum shift in mean July temperature was at that time c. +2.5.” Get copy here.

Solovieva and Jones studied a multi-proxy record of the Kola Peninsula in northern Russia and concluded that for the period from 8000 years ago to 5400 years ago “A maximum of forest cover and the high Pinus abundance during this period indicate the Holocene climate optimum. The multiproxy data from Chuna Lake generally agree with the temperature reconstructions based on the evidence from the Greenland ice-cores (Stuiver et al., 1995) and summer temperatures were likely to have been 2°–3 °C higher than at present.” Get copy here.

MacDonald, et. al., dated Scots Pine wood (Pinus sylvestris L.) in Russia’s Kola Peninsula and found “the density of trees north of the modern tree-line was greatest between 7000 and 5000 BP. Get copy here.

“Pollen, stomata, and macrofossils in a lake core with a basal date of 9700 14C BPwere used to reconstruct past changes in climate and vegetation in the arctic tree line area, northeast European Russia” … “We interpret summer temperatures to have been ca. 3–4 °C higher between ca. 8900 and 5500 BP than at present, and the lowest temperature regime of the Holocene to have occurred between 2700 and 2100 BP.”

Koshkarova and Koshkarov(2004) draw their conclusions based on “25 sections of Holocene deposits and soils of northern Central Siberia [that] were studied by paleocarpologicalmethods. Special attention was given to the reconstruction of the dynamics of speciation of forest cover in time and space.” These 25 sections are all above the arctic circle and range in longitude from 86 to 119°E. They divide the Holocene in the region into “intervals 9-8 ka (thermal maximum), 6.5-5ka (climate optimum – combination of higher temperatures and higher humidity), and 2.5-2 ka (thermal minimum). Get copy here.

Monserud, et. al., concentrated on the mid-holocene, which they defined as 4600 to 6000 years before the present. They found that during this period the Siberian winters “between 60 and 65N the palaeoclimate was 5.3 C warmer on average, and between 65 and 70N it was 7.7 C warmer.” For the warmer months the found “Summer was 2-5 C warmer than today between 63 and 73N, embracing much of the Northern Taiga, Forest-Tundra, and Tundra zones. A band of moderate summer temperature anomalies (0 – 2 C) is centered at 65N, and a second band of greater anomalies (2-5 C) is centered at 70N.” Get copy here.

Ilyashuk, et. al. show that Radiocarbon-dated chironomid records from the lake Nikolay region of the Lena River Delta area “imply the warmest (up to 2-3°C warmer than nowadays) climate during ca. 10,200-9200 cal. yr BP…with two short warm oscillations (up to 8.9oC) at c. 5600 and 4500-4100 cal. yr BP…and a relatively long warm period ca. 2300-1400 cal. yr BP.” Get copy here.

Matul, et. al., (2007) from the Russian Academy of Science studied microfossils from the Laptev Sea, which is north of Siberia and well within the Arctic circle. They found that “Judging from the increased diversity and abundance of the benthic foraminifers, the appearance of moderately thermophilic diatom species, and the presence of forest tundra (instead of tundra) pollen, the Medieval warming exceeded the recent “industrial” one and is reflected in the near-delta sediments.” But they indicate that it was warmer even earlier by saying “..the warming in the Laptev Sea during the period of ~5100–6200 years B.P. corresponding to the Holocene climatic optimum could be even more significant as compared with the Medieval Warm Period.”

Lawson looked at glacial advances and retreats in Glacier Bay, Alaska. Glacier Bay is well south of the Arctic circle, but yields information about northern latitude climates. They found a glacial retreat starting 6800 years ago followed by a new glacial advance starting 5000 years ago. The retreat “was long enough to develop a mature forest” on land that was subsequently recovered with ice. Get cop here.

In a very comprehensive study of the western Arctic Kaufman and coauthors from the US, UK, Canada, Norway, Iceland, and Russia (2004), studied proxies from over 140 sites in the western hemisphere part of the arctic. Their abstract notes “Paleoclimateinferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrialsites where quantitative estimates have been obtained, local HTM[Holocene Thermal Maximum] temperatures (primarily summer estimates) were on average 1.6 ± 0.8 ° C higher than present…”

They devided the region into four zones, which I have labeled on the map.

12a. Central Eastern Beringia.Sketchy evidence indicates that the Holocene Therma Maximum occurred very early and had a short duration in this region. Temperatures were several degrees above current temperatures for some period between 12.8 and 7.1 ka. (mean initiation plus one sigma to mean termination minus one sigma).

Stewart and England examined more than 70 samples or Holocene driftwood on Ellesmere at 82° N Latitude. The time distribution of the driftwood indicates “prolonged climatic amelioration at the highest terrestrial latitudes of the northern hemisphere” from 4200 to 6000 years before the present. Get copy here.

“Dahl-Jensen, et. al., use borehole data to conclude “After the termination of the glacial period, temperatures in our record increase steadily, reaching a period 2.5 K warmer than present during what is referred to as the
Climate Optimum (CO), at 8 to 5 ka. Following the CO, temperatures cool to a minimum of 0.5 K colder than the present at around 2 ka. The record implies that the medieval period around 1000 A.D. was 1 K warmer than present in Greenland.” Get copy here